Ultrasound has been used since the 1940's to non-destructively inspect a wide variety of materials for flaws, phase constitution, dimension measurement, grain structure and integrity. In particular, modern nondestructive examination (NDE) methods typically utilize sonic energy in the megahertz range to penetrate and image the inner body of metals, as well as their outer surfaces, taking advantage of their acoustical properties in locating discontinuities that reflect or scatter acoustical waves. The reflective property of voids, flaws, cracks, etc., that could be detrimental to the continuity and integrity of the material is the basis of NDE methodology. The frequency used is determined by the type of material and technique employed; for steel it is in the range of 1-10 megahertz with 2.25 to 5 megahertz the preferred range set by the propagation and attenuation characteristics of various steels. Other frequencies are used for zircaloy, titanium, aluminum and composite materials as dictated by their particular acoustic properties.
Typically, ultrasonic waves generated by a piezoelectric crystal transducer, common and known to the art, are introduced via a coupling fluid, such as water or acoustical grease, at the surface of the metal to be inspected. As the waves propagate in the bulk of the material they may impinge on some type of discontinuity affecting the acoustical impedance of the medium. It is well known in the science of acoustics that this impingement produces reflections and transmissions that compete against each other, depending on various factors such as flaw size and shape, angle of incidence, and magnitude of the change in impedance. In case of a gas gap (usually and naturally filled with air) change of impedance is so abrupt and large that virtually all of the incident sound waves are reflected at the interface. Very little sonic energy traverses such a gap, and inspection of material beyond an air gap is never considered in NDE practice. Thus, in many applications important to nuclear plant component inspections NDE effectiveness is limited by the presence of gaps that shield important joints and zones from inspection. An example is that of the control rod drive housing to stub-tube attachment weld and heat affected zone, known to be subject to cracking.
Referring to FIG. 1A, a reactor vessel V is shown in partial section to display a core C. Core C contains control rods, whose drive housings H extend through the bottom of the vessel V through stub-tubes T. Those familiar with the nuclear industry will recognize that FIG. 1A is a boiling water reactor operating under a standard pressure in the range of 1200 pounds. Further, the vessel is in the range of 120 feet in height, 30 feet in diameter, and contains radioactive material contained in fuel rods as the natural result of the nuclear reaction. The preferred nondestructive examination is directed at the stub-tube environment which is at the bottom head of vessel V.
Referring to FIG. 1B, bottom head 14 of the vessel V is illustrated at stub-tube T placed within a recessed aperture 15 of vessel V. Typically, the inside of vessel V can be clad with stainless steel 16. Connection of the stub-tube T to the cladding 16 and vessel V at bottom head 14 occurs at weld L. Similarly, connection of the control rod drive housing H occurs at the top of the stub-tube T.
It is required that the alignment of the control rod drive housing H within the stub-tube T be precise. Consequently, the stub-tube T and the control rod drive housing define a gap G therebetween. This gap G enables the verticality of the control rod drive housing H to be maintained during the placement of welds J.
Welds J and L, and the heat-affected zones adjacent to the welds are subject to certain conditions of metal cracking. Specifically, these zones have proven to be candidate zones for the metallic cracking defect known as intergranular stress corrosion cracking (IGSCC). Simply stated, the conditions of metallic tension, stagnation of water flow, and oxygen concentration cause crack propagation along the granular boundaries of the metal. This phenomenon is known to occur within and adjacent to welds L and J.
Before this disclosure, gap G prevented ultrasound examination of cracking in stub-tube T from the interior of control rod drive housing H.